1. Field of the Invention
This invention relates to a commercial process for the preparation of 2,5-hexanedione, i.e., acetonylacetone, from allylacetone and, more particularly, relates to the oxidation of allylacetone using palladium chloride as a catalyst in the presence of copper chloride and oxygen, whereby high yields of 2,5-hexanedione are prepared with only small losses of the palladium catalyst.
2. Description of the Prior Art
2,5-Hexanedione or acetonylacetone is important as an organic chemical intermediate. Preparation of this compound through various synthesis routes has been reported in the prior art. For example, Adams et al. in J. Am. Chem. Soc., Vol. 72, p. 4368 (1950), describe the synthesis of 2,5-hexanedione by condensing propylene oxide with acetoacetic acid esters to produce alpha-aceto-gamma-valerolactone, which, in turn, is reacted with dilute hydrochloric acid and converted into 5-hydroxy-2-hexanone. To obtain 2,5-hexanedione, the hydroxy-hexanedione product is then oxidized together with sodium dichromate and sulfuric acid. Also, Shenk in Ber., Vol. 77, p. 661 (1944), describes the preparation of 2,5-hexanedione by oxidizing 2,5-dimethylfuran to 3-hexene-2,5-dione, which product is then hydrogenated to produce 2,5-hexanedione. Still further, in U.S. Pat. No. 2,525,672, Heilbron et al. describe the preparation of 2,5-hexanedione by first reacting 1-bromo-2,3-epoxy-butane with monosodium acetylide in liquid ammonia, and then reacting the 3-hexene-5-yn-2-ol product obtained with mercury sulfate in sulfuric acid.
More recently, in Kogyo Kakaku Zasshi, 71, (6), p. 945-6 (1968), as well as in Japanese Patent Publication No. 1972-11411, Takamori Konaka and Sadao Yamamoto have described a simplified, 1-step process for producing good commercial yields of 2,5-hexanedione from allylacetone in a mixed solvent system which is composed of water in combination with either benzene or dimethylformamide. Palladium chloride is employed as the oxidation catalyst in the presence of prescribed amounts of cupric chloride and oxygen. This process is carried out usually at temperatures of 60.degree.-80.degree. C for overall time periods ranging generally from 3 to 12 hours but typically from 7 to 12 hours. Upon completion of the reaction, the 2,5-hexanedione product is reported as being easily recoverable from the reaction mixture and purified. However, from practice of this process, substantial quantities of undesirable byproducts oftentimes are obtained and losses of the expensive palladium chloride catalyst component are found to be substantial.
It has now been found that by conducting the oxidation process in a manner similar to that described in the aforesaid Japanese patent publication, but with the use of a mixed solvent system composed of water and carbon tetrachloride, commercially acceptable yields of 2,5-hexanedione can be conveniently obtained in shorter reaction times and with much reduced losses of the palladium catalyst.